Optical fiber transmission systems often require that the signal carried through the system be reduced or attenuated before it can be used. Such attenuation generally is carried out using either an attenuator that can be adjusted in situ to provide a desired attenuation level, or in the alternative, by using an attenuator having a fixed, predetermined attenuation factor that is selected prior to use for the specific application. The latter type of attenuator is referred to herein as a "fixed value fiber optic attenuator". The majority of the fixed value attenuators currently used in fiber optic transmission systems rely upon passive filters, air gaps, optical wedges and/or lenses to reduce the signal strength in optical transmission lines.
With increasing data transmission speeds, a phenomenon referred to as "back-reflection" or "return loss" has been identified as causing significant losses in signal strength and quality. The phenomenon is characterized by Fresnel reflections which occur when light is transmitted from a first region having one index of refraction to a second region having a different index of refraction.
Current optical fiber attenuators generally operate by employing materials having refraction indices that differ from the refraction index of the optical transmission fiber to thereby cause a reduction in optical signal strength. Thus, back-reflection, and its resultant signal deterioration, is an inherent characteristic of most attenuation systems currently in use.
In addition, most of the attenuators used in fiber optic transmission systems rely upon materials that provide a differing degree of attenuation for differing wavelengths of light. Thus, it becomes necessary to identify the wavelengths to be utilized in the transmission system prior to selecting the specific attenuators. Otherwise, if the wavelengths carried through the system vary, the attenuation will likely vary as well.
U.S. Pat. No. 5,109,468, assigned to the assignee of the present invention, is directed to a "fixed value fiber optic attenuator". The fiber optic attenuator described in that patent, as shown particularly in FIGS. 2(a)-(b) of that patent and the corresponding drawings in the present application, provides attenuation by dividing, bending or otherwise altering a fiber optic pathway having an attenuation region which causes a predetermined signal loss. However, it has been found by the inventors of the present application that even with the invention of the U.S. Pat. No. 5,109,468 a certain amount of cladding "noise" or modal "noise" is introduced. It was found that the attenuator described in U.S. Pat. No. 5,109,468 was largely of use only in those applications in which the hardwared attenuated fiber shown in FIG. 1 (28) could be used in optical power transmission of the same fiber type as the attenuated fiber 28 and that the attenuator was largely signal wavelength dependent. Thus, if the hardwared attenuated fiber shown in FIG. 1 of the patent were used in the fiber optic power transmission system having a detector side optic fiber of core diameter larger than that of the attenuated fiber, cladding modes located in the cladding could be transmitted into the core of the larger diameter fiber, thereby allowing more of the power transmission through the fiber optic transmission system than the hardwared attenuator fiber was originally designed to attenuate. In the device described in the above patent, the cut fiber of FIG. 2(a) and the microbent (or stressed) fiber shown in FIG. 2(b) introduce a controlled amount of "cladding light" to effectively attenuate the power transmission through the fiber's core. Typically, these cladding light modes are stripped out when an additional length of single mode fiber (core diameter approximately 9 micrometers) is added to the detector side of the system.
Because of the introduction of cladding modes in a single mode fiber of the device shown in the above patent, a length of singlemode type fiber (ranging from one-third meter to two meters in length typically) is required to effectively "strip out" these cladding modes, allowing the detector to "see" only the core mode. It is these deficiencies of the prior art device described in U.S. Pat. No. 5,109,468 that the present invention attempts to overcome.